The basic image-forming process of color photography comprises the exposure of a silver halide photographic recording material, such as a color film, to light, and the chemical processing of the exposed material to provide a useful image. The chemical processing involves two fundamental steps. The first is a treatment of the exposed silver halide material with a color developing agent wherein some or all of the silver ion is reduced to metallic silver and a dye image is formed.
The second fundamental step is the removal of silver metal by one or more steps of bleaching and fixing so that only a dye image remains in the processed material. During bleaching, the developed silver is oxidized to a silver salt by a suitable bleaching agent. The oxidized silver is then dissolved and removed from the element using a "fixing" agent or silver solvent in a fixing step.
The most common bleaching agents are complexes of ferric ion and various organic ligands (such as aminopolycarboxylic acids), of which there are hundreds of possibilities, all with varying bleaching activities and biodegradability. Common organic ligands used as part of bleaching agents for color film processing include ethylenediaminetetraacetic acid (EDTA), propylenediaminetetraacetic acid (PDTA), methyliminodiacetic acid (MIDA) and nitrilotriacetic acid (NTA).
U.S. Pat No. 4,294,914 (Fyson) describes bleaching and bleach-fixing compositions and a processing method using a ferric complex of one of several alkyliminodiacetic acids, which are known to be more biodegradable than other common organic ligands such as EDTA. Other bleaching agents using similar organic ligands are described in U.S. Pat. No. 5,061,608 (Foster et al) in which the bleaching agent is advantageously combined with specific aliphatic carboxylic acids to reduce dye stains. U.S. Pat. No. 5,334,491 (Foster et al) also describes the use of similar biodegradable bleaching agents in combination with specific levels of bromide ion.
U.S. Pat. No. 5,508,150 (Craver et al) describes another solution to the problem of iron stain. Bleaching is carried out using a bleaching agent that is ferric ion chelated with a tridentate or tetradentate ligand. MIDA is an example of a tridentate ligand. Bleaching is followed by fixing with a composition comprising an uncomplexed polycarboxylate.
In processes utilizing methyliminodiacetic acid (MIDA) as the bleaching agent ligand, iron-MIDA dissociation may occur in the wash bath. This rust formation must be controlled by preventing the dissociation of MIDA from ferric ion. In copending and commonly assigned U.S. Ser. No. 08/795,885, filed Feb. 6, 1997, by Foster, a method of photographic processing is described wherein "anti-rust" agents are included in the bleaching solutions in order to control the dissociation of MIDA and similar ligands from ferric ion. Such agents are defined as organic phosphonic or phosphinic acids or salts thereof and were found to be highly effective.
However, during an investigation of various biodegradable bleaching compositions including those containing MIDA, a severe yellow stain from retained iron was observed in some processed photographic elements. Various additives have been tested to eliminate such stains.
For example, DE 4,226,372 (Tappe et al) describes bleaching solutions containing excess .beta.-alaninediacetic acid (ADA) and a hydroxycarboxylic acid additive, such as citric acid or tartaric acid, to reduce the precipitation of iron hydroxide in the wash bath following bleaching. However, such additives are not effective with the use of every biodegradable bleaching agent including the use of MIDA.
Thus, there is continuing need for a means to reduce or eliminate iron stain in all processed photographic elements, especially those processed using biodegradable bleaching agents.